Microelectronic and micromechanical devices are generally formed on a semiconductor wafer, a glass substrate or another type of workpiece using several repetitions of doping, deposition, etching, planarizing and/or lithographic processes. A plurality of individual devices are fabricated on a single workpiece and then the workpiece is cut to form multiple individual dies, such as semiconductor integrated circuit (“IC”) chips, field emission displays and other devices. Each chip includes many semiconductor components, conductive lines, etc. The devices accordingly include various materials, including electrical conductors (e.g., aluminum, tungsten, copper), electrical semiconductors (e.g., silicon) and electrical non-conductors (e.g., silicon dioxide).
Lithographic processes delineate a pattern in a layer of material (e.g., photoresist) sensitive to photons, electrons, or ions. The principle is similar to that of a photocamera in which an object is imaged on a photo-sensitive emulsion film. Unlike a photo-camera where the “final product” is the printed image, the image in the semiconductor process context typically is an intermediate pattern that defines regions where material is deposited on or removed from the wafer. Lithographic processes typically involve multiple exposing and developing steps. An exposing step typically involves directing a patterned beam of photons. electrons, or ions against the layer of resist. After an exposing step, a developing step involves removing one of either the exposed or unexposed portions of photoresist. Complex patterns typically require multiple exposure and development steps.
A typical lithographic system, shown in FIG. 1, includes a light source 12, an optical system 14, and a transparent photomask 15. During a lithographic process, light 13 from the light source 12 passes through the optical system 14 and the photomask 15 onto a photoresist layer 18. The photomask 15 is held in place by the reticle table 16. The controller 20, also known as the aligner, controls the relative position of the light source 12 and reticle table 16. The photomask 15 defines the “intermediate pattern” used for determining where photoresist is to be removed or left in place. Conventional photomasks generally have a transparent glass blank and a thin opaque film on the blank. The blank and the film together define a reticle. Conventional materials for the blank include soda lime, borosilicate glass, or fused silica. The opaque film can be a patterned layer of chrome less than 100 microns thick and an anti-reflective coating, such as chrome oxide, on the chrome. The purpose of the anti-reflective coating is to suppress ghost images from the light reflected by the opaque material.
FIG. 2 shows a conventional reticle 14 including a transparent plate or “blank” 22 covered with a patterned opaque film 24. The recticle 14 has a mask area 26 in which the film 24 includes several masks 28 or patterns on respective portions of the photoresist layer 18. Each mask 28 within the mask area 26 may be different, so as to make different integrated circuits. Hereafter, the terms “pattern”, “mask”, and “photomask” are used synonymously in both the singular and plural sense and should not be construed to limit any aspect of the description. The patterned opaque film defines the pattern that will be formed in the resist layer for depositing, etching, doping or implanting processes on the wafer. The patterned film on the reticle blank includes mask lines and line spaces that are typically less than 10 microns. Depending on a reduction factor, the line width and line space geometry for a resulting semiconductor device are much smaller than the mask lines and line spaces on the patterned film. Other mask line spacing and semiconductor line spacing can also be achieved. When working with a high density of small features, it is important that the reticle and other components in the fabrication processes be free of foreign particles. A tiny speck of dust, for example, can alter the desired pattern being imaged onto the wafer.
One conventional technique for protecting the photomask on a reticle is to cover the photomask with a thin transparent membrane, referred to as a pellicle membrane. FIGS. 2 and 3 show a pellicle frame 30 that encloses the mask area 26 of the reticle having the masks 28. A thin membrane called a pellicle membrane 32 is adhered to the pellicle frame 30. The pellicle membrane 32 protects the mask area 26 from the external environment and is sealed to the pellicle frame 30 using a pellicle glue ring. The pellicle membrane 32 is positioned at a height above the photomask greater than the focal length of the light imaged onto the photomask. Thus, small particles on the pellicle membrane will not block light from reaching the photomask.
When a reticle with masks 28 is formed, the surface is cleaned and qualified to insure that the mask is accurate and that no foreign particles are present. As part of such qualification process, the pellicle membrane 32, which is formed of conventional materials such as cellulose acetate or nitrous cellulose, is adhered to the pellicle frame 30 protecting the masks 28 from foreign particles.
One problem caused by foreign particles even when the reticle is protected by the pellicle membrane 32 is that the reticle may not be registered properly relative to the table or another reticle. As shown in FIG. 1, the reticle 14 rests on a reticle table 16 during the lithographic process. The lithographic processes often require that a given reticle 14 be replaced on the reticle table 16 with another reticle having a different mask pattern. This movement of reticles on and off the reticle table 16 can cause small particles to adhere to the reticles 14. Further, reticles typically are stored in a carrying case. Microscopic particles also may adhere to the reticles from rubbing along rails of the reticle carrying case. If any foreign particles are on the reticle in the regions 33 where the reticle 14 contacts the reticle table 16, then the reticle may not be seated for proper alignment. A foreign particle under a portion of the reticle 14, for example, may cause that portion of the reticle 14 to be higher than another portion. This can cause inadequate registration of the light passing through a mask onto a wafer, or misalignment from one mask to another mask. If such problems are detected, the reticle must be removed and cleaned.
Another problem of photo-lithography is that cleaning the reticle can be difficult. For example, cleaning the reticle is time consuming, which can cause significant downtime for a stepper machine and affect the throughput of workpieces. Additionally, because the pellicle membrane 32 typically is very fragile, it may be destroyed during the course of cleaning the reticle. A typical procedure for cleaning a reticle involves removing the pellicle membrane 32 from the pellicle frame 30 to clean the surface of the reticle. The pellicle glue ring is also removed in this process. The solvent necessary to remove the glue ring is, however, caustic and can irreparably damage the photomask portion of the reticle. Thus, even when reticles are handled with great care, the photomask portion is easily damaged merely by the chemicals that are necessary for preparing the pellicle frame for a new pellicle membrane.